1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber, and to an optical fiber.
2. Description of the Related Art
In optical fibers generally used in optical transmission systems, it is necessary to guarantee long-term mechanical reliability, for example, for 20 years, in the usage environment. Therefore, mechanical reliability (lifetime and failure probability) is calculated for an optical fiber based on the measured fatigue coefficient and screening strength of the optical fiber and strain to be applied to the optical fiber in the usage environment. In order to evaluate the mechanical reliability, the value of static fatigue coefficient or dynamic fatigue coefficient, screening strength, and failure frequency are mainly used, whereas failure time and failure strength are not used.
In recent years, since fiber-to-the-home (FTTH) networks have been developed, installation of optical fiber transmission lines for access systems has been increasing. In access systems, it is difficult for communication carriers to control installation. Furthermore, there have been increased chances for users to handle optical fibers in their premises or the like. In such access systems or premises, there is a concern that optical fibers may be kinked or stepped on during installation and handling, and thus excessive strain may be temporarily applied to optical fibers.
As described in W. Griffioen, et al, “Reliability of Bend Insensitive Fibers,” Proceedings of the 58th IWCS/IICIT, pp. 251-257 (2009), it is known that in the case where an optical fiber is bent around a small radius (e.g., with a bending radius of 2.4 mm or less), the intrinsic strength of the glass of the optical fiber influences the failure probability. However, an optical fiber whose failure time is guaranteed when it is bent around a small radius, and a method of manufacturing the same are not known.
Japanese Unexamined Patent Application Publication Nos. 6-250053 and 2005-55779 each describe that, by controlling the pH of a fiber coating, the fatigue coefficient of the optical fiber can be improved. However, the fatigue coefficient cannot always be improved by controlling only the pH. Furthermore, failure strength and failure time at the time of being bent around a small radius are not examined in these documents.
Japanese Unexamined Patent Application Publication No. 2006-215445 describes that, by setting the tetraethoxysilane content to be 0.1 parts by mass to 3.0 parts by mass and the diethylamine content to be 0.001 parts by mass to 0.2 parts by mass in a primary coating which is a part of a fiber coating and is in contact with the outer circumference of a glass fiber, it is possible to prevent a reduction in strength of the glass fiber in the process of cleaning the surface of the glass fiber after the fiber coating has been removed. However, by adding an amine-based additive, such as diethylamine, to the primary coating, the basicity of the photo-cured primary coating is increased, and as a result, long-term reliability of the optical fiber is rather decreased. Furthermore, failure strength and failure time at the time of being bent around a small radius are not examined in this document.
As described above, there has not been a sufficient accumulation of knowledge on failure time when optical fibers are bent around an excessively small radius. An optical fiber whose failure time is guaranteed at the time of being bent around a small radius or a method of manufacturing the same is not known. Furthermore, failure strength and failure time at the time of being bent around a small radius have not been examined.